The present invention relates to a variable magnification copying lens system that is useful in the optics of a copying machine and which is capable of realizing both size enlargement and reduction while maintaining a constant distance between the object surface and the image surface.
The conventional variable magnification lens systems capable of size enlargement and reduction while maintaining a constant distance between the object and image are classified into the following three types:
(i) a system that is simply composed of two negative and positive, or positive and negative, lens groups, but which is incapable of attaining a zoom ratio of 2 (see, for example, Japanese Patent Publication No. 13887/1983 and Unexamined Published Japanese Patent Application No. 68810/1982);
(ii) a system that is composed of three negative, positive and negative lens groups, or four negative, positive, positive and negative lens groups, but which is incapable of attaining a zoom ratio much greater than 2 (see, for example, Unexamined Published Japanese Patent Application Nos. 159614/1981, 67909/1982 and 67512/1984); and
(iii) a system that is composed of three negative, positive and negative lens groups, or four negative, positive, positive and negative lens groups, and which is capable of attaining a zoom ratio as great as 4 or 9, but which is useful in a variable magnification lens system for platemaking, rather than copying, purposes because of the large object-to-image distance (small view angle), the large size of the overall lens system and the great number of lens elements incorporated (see, for example, Unexamined Published Japanese Patent Application Nos. 1242/1974, 60655/1978 and 11260/1980).
However, these conventional lens systems are either low in zoom ratios (a little more than 2) or bulky and are incapable of meeting the demands of recent versions for relatively high zoom ratios and compact arrangements.
The conventional systems consisting of three or four lens groups use a master lens group (reference lens group) with six or more lens elements arranged in a substantially symmetrical fashion, and in some of the systems, the aerial distance in the center is fixed, but in most systems, the central aerial distance is variable and requires complicated lens arrangements and zooming methods.